1. Field of the Invention
The present invention generally relates to a method of positioning a sound image and a sound image positioning apparatus for use in a three-dimensional sound system or else. More particularly, the present invention relates to a method of simulating acoustic transfer characteristics from a virtual sound source in a virtual sound field.
2. Description of Related Art
In a three-dimensional virtual reality system for example, a sound image positioning apparatus is conventionally used as a means for enhancing presence of virtual reality experience. In such a system, a cubic sound field is generated by creating direction perspective and distance perspective in auditory sensation by producing audio signals from a monaural sound source through a plurality of channels having time difference, amplitude difference, and frequency characteristic difference based on binaural technique. To be more specific, an input audio signal is attenuated in a particular frequency component by a notch filter, for example, to create elevation. The input audio signal is also converted by a delay circuit into left channel and right channel signals having a time difference, and is further given acoustic transfer characteristic from a virtual sound source by a FIR (Finite Impulse Response) filter. A parameter of the FIR filter is given from an HRTF database storing head-related transfer functions (HRTF) measured with using a dummy head in advance.
In the above-mentioned conventional sound image positioning apparatus, it is impracticable to store the HRTFs corresponding to all virtual sound source points included in a sound field. Normally, only the transfer characteristics at points radially away from a listener by a certain distance, for example one meter, are measured and stored. Therefore, if a virtual sound source is located one meter away from a listener as shown in FIG. 5, proper sound image positioning can be provided. However, if the virtual sound source is located from the listener at a distance more or less one meter, problem occurs that sound images sensed by the right and left ears of the listener do not match each other, losing good positioning. Especially, it is known that the human ear has an angular resolution of xc2x13xc2x0 for acoustic direction. Therefore, if a virtual sound source passes across the listener, an error higher than xc2x13xc2x0 may occur, thereby causing a sense of incongruity.
It is therefore an object of the present invention to provide a sound image positioning method and a sound image positioning apparatus for positioning a sound image to a correct point even if there is a difference between a reference distance used for measuring a head-related transfer function stored beforehand and a setting distance of a virtual sound source.
The inventive method pans a sound image of a virtual sound source to a listener in a virtual sound field by filtering audio signals of left and right channels through left and right filters which simulate acoustic transfer characteristics of the virtual sound field. The inventive method comprises the steps of provisionally memorizing acoustic transfer characteristics of the virtual sound field which are distributed radially around a center point of the listener, designating a source point at which the virtual sound source is to be located within the virtual sound field in terms of a geometric distance and a geometric direction relative to the center point, computing a leftward acoustic direction from the source point to a left ear of the listener according to the geometric distance, the geometric direction and an offset of the left ear from the center point, computing a rightward acoustic direction from the source point to a right ear of the listener according to the geometric distance, the geometric direction and an offset of the right ear from the center point, determining an effective acoustic transfer characteristic based on the memorized acoustic transfer characteristics according to the leftward acoustic direction so as to enable the left filter to simulate said effective transfer characteristic, determining another effective acoustic transfer characteristic based on the memorized acoustic transfer characteristics according to the rightward acoustic direction so as to enable the right filter to simulate said another effective transfer characteristic, and filtering an audio signal of the left channel through the left filter and filtering another audio signal of the right channel through the right filter to thereby direct the sound image of the virtual sound source located at the source point to the listener positioned at the center point.
Further, the inventive apparatus is constructed for directing a sound image of a virtual sound source at a designated source point to a listener in a virtual sound field. In the inventive apparatus, a database provisionally memorizes acoustic transfer characteristics of the virtual sound field in correspondence to reference source points distributed radially around a center point of the listener. Left and right filters respectively filter audio signals of left and right channels according to the acoustic transfer characteristics loaded from the database. A processor computes a leftward acoustic direction from the designated source point to a left ear of the listener, and computes a rightward acoustic direction from the designated source point to a right ear of the listener. A controller specifies a leftward reference source point coincident with the leftward acoustic direction to load an effective acoustic transfer characteristic corresponding to the leftward reference source point from the database into the left filter, and specifies a rightward reference source point coincident with the rightward acoustic direction to load another effective acoustic transfer characteristic corresponding to the rightward reference source point from the database into the right filter. A feeder feeds an audio signal of the left channel to the left filter and feeds another audio signal of the right channel to the right filter to thereby direct the sound image of the virtual sound source located at the source point to the listener positioned at the center point.
In a different view, the inventive apparatus is arranged for directing a sound image of a virtual sound source to a listener in a virtual sound field. In the apparatus, a database provisionally memorizes a pair of leftward and rightward acoustic transfer characteristics of the virtual sound field in correspondence to each of sample points distributed radially around a center point of the listener at a fixed radius, the leftward acoustic transfer characteristic simulating a path from each sample point to a left ear of the listener and the rightward acoustic transfer characteristic simulating another path from each sample point to a right ear of the listener. Left and right filters respectively filter audio signals of left and right channels according to the left and right acoustic transfer characteristics loaded from the database. An input designates a source point at which the virtual sound source is to be located within the virtual sound field in a distance which may be different from the fixed radius relative to the center point. A processor computes a leftward acoustic direction from the source point to the left ear of the listener, and computes a rightward acoustic direction from the source point to the right ear of the listener. A controller specifies a leftward sample point substantially coincident with the leftward acoustic direction to load the leftward transfer characteristic corresponding to the leftward sample point from the database into the left filter, and specifies a rightward sample point substantially coincident with the rightward acoustic direction to load the rightward acoustic transfer characteristic corresponding to the rightward sample point from the database into the right filter. A feeder feeds an audio signal of the left channel to the left filter and feeds another audio signal of the right channel to the right filter to thereby direct the sound image of the virtual sound source located at the source point to the listener positioned at the center point.
According to the present invention, as shown in FIG. 3, based on the geometric distance r and geometric direction xcex8 to the virtual sound source point Ps and the offset 2h between both ears of the listener, the acoustic directions R and L from the virtual sound source point Ps to the right and left ears of the listener are calculated separately for the right and left channels. Acoustic transfer characteristics of the right and left filters are determined by these acoustic transfer directions R and L. To be more specific, sample points PR and PL coincident with the acoustic directions R and L are identified on a circumference having the fixed radius r0 that is the reference distance. If the setting distance r to the virtual sound source point Ps differs from the reference distance r0 at which the acoustic transfer characteristics are measured, the acoustic transfer characteristics corresponding to the sample points PR and PL are used as the effective acoustic transfer characteristics of both channels. Approximating the effective or true characteristics from the object distance r by these acoustic transfer characteristics provides high fidelity of the sound image positioning. The above-mentioned acoustic transfer characteristics are stored in the transfer characteristic database beforehand at a fine angular pitch, for example, in unit of 1xc2x0 on the circumference away from the listener by the fixed reference distance r0. Alternatively, the data may be stored at a coarse angular pitch, for example, in unit of 90xc2x0 in forward, backward, rightward and leftward directions. In the latter case, the effective acoustic transfer characteristic corresponding to the acoustic direction concerned may be obtained by vector compoint or composition according to the calculated acoustic directions R and L. Consequently, according to the present invention, an acoustic transfer characteristic of higher fidelity can be obtained with generally the same data volume as that used in the prior art technology. Conversely, a smaller data volume than that conventionally used may be enough for achieving generally the same acoustic transfer characteristic as that of the prior art technology.